How we build and verify our calculators
Correctness is the whole point of this site, so here is how a FinExplained calculator gets built, tested, and kept right over time. This is the process; for the actual formula and a worked example behind any single tool, see our methodology and that calculator's own page.
One shared engine, no copy-pasted math
Every calculator imports its formulas from a single shared engine. A mortgage payment, an amortization schedule, or a tax bracket walk is defined once and reused everywhere, so the same input always produces the same result and no formula can drift out of sync between two pages.
Decimal-precise money math
All money math uses a decimal library, not ordinary binary floating point, so cents add up exactly and the rounding errors that creep into spreadsheets do not appear. Numbers are formatted for display only at the very end, using the Intl standard, so what you read matches what was computed.
Assumptions traceable to the code
Each calculator lists its assumptions on the page, and every assumption is derived from what the engine actually does, not a generic disclaimer. When we change an input, an output, or a default, we update the stated assumptions to match, so the copy never claims behavior the code does not have.
Tested to the cent
Each calculator ships tests that check it against known-correct results to the cent, plus property tests that must always hold (for example, that the principal portions of an amortization schedule sum exactly to the loan). A wide fuzz suite also round-trips thousands of randomized inputs through every calculator to confirm results stay stable. A formula that fails a test does not ship.
Automated gates on every change
Before anything is published, every change has to pass the full test suite, a type check, and a linter. The build then runs a set of guards that fail it on a regression: a content-style guard that enforces our plain-language copy rules, a URL guard that checks every internal link, and a security guard that verifies the page's content-security policy. If any gate is red, the change does not go out.
Annual review of year-stamped figures
Figures that change each year (tax brackets, the standard deduction, contribution limits, the Social Security wage base) are held in one place and reviewed on a yearly checklist. The calculators always apply the current year's figure, and year-stamped pages note which tax year they describe so nothing silently goes stale.
Where the numbers come from
The figures behind the math come from primary sources: the IRS, the SSA, the BLS, and other official bodies. We list each one, and what it backs, on our sources page.
For education, not financial advice. See our disclaimer and our editorial standards.